1. Field of the Invention
This invention relates to moldable granular compositions that are self-setting under ambient conditions. The invention finds general application in the production of shaped articles such as grinding wheels, electrical or thermal insulators, bricks and similar structures of bonded granular composition, but is particularly concerned with foundry sand compositions, and their method of preparation, which are self-setting in a controllably short time under ambient atmospheric conditions, and do not require baking or firing to develop adequate strength and rigidity to receive molten metal, yet are effective to provide flaw-free surfaces on finished castings.
The novel compositions are notably free of deleterious elements that could cause pollution of the atmosphere due to evolution of toxic or hazardous gases during cure or when the shapes are subjected to high temperatures as in casting of molten metal into foundry molds made of the compositions. The compositions are also free of contaminating elements which, if present in foundry mold or core surfaces, have a tendency to migrate into the adjacent molten metal at the interface with the mold during the casting process, thereby adversely affecting critical alloy compositional requirements of the final castings.
The principal current interest in the invention is its application to the foundry art for providing self-setting molds and cores having substantial advantages over prior systems in terms of meeting environmental, material and energy cost, and high rate of production, considerations. Accordingly the invention is discussed principally herein with relation to its foundry use, but as will be apparent it has many other applications.
2. Description of the Prior Art
Trends in the foundry industry in recent years have been toward processes which minimize labor requirements, both from the standpoint of quantity and skill level, are low energy consumers, and are capable of high production rates. To meet these requirements, a number of no-bake or air-setting binder systems have been developed that, when combined with some of the newer continuous mixers available on the market, are capable of producing molds and/or cores which can be stripped from the pattern as a rigid, self-sustaining sand mass in as little as 11/2 to 2 minutes. Molds and cores made from these systems are relatively erosion resistant to molten metal and produce castings of superior dimensional reproducibility over those employing conventional green sand molding practices.
Particulates such as silica sand, clay and the like are bonded together to form the desired molds or other shapes. A wide variety of chemical binder systems are commercially available and are being used routinely in steel foundries. These systems are considered self-setting in nature in that they require no external heat source to effect the bond. The following tabulation outlines those available binder systems, categorizing them chemically as either inorganic or organic in origin:
______________________________________ Summary of Available Self-Setting Binder Systems ______________________________________ I. Inorganic A. Cement 1. Portland 2. Modified Portland 3. Calcium Aluminate B. Sodium Silicate 1. Cement 2. Sodium Silicofluoride 3. Silicon Metal or Ferrosilicon 4. Organic Ester C. Polyphosphate II. Organic A. Furan 1. High Nitrogen 2. Medium Nitrogen 3. Phenol-Formaldehyde B. Alkyd Isocyanate C. Phenolic D. Phenolic Urethane ______________________________________
In the case of inorganic foundry mold mix systems, which are of more particular interest here, the available systems have evolved over the years into what may be generally classified as three basic types, these being identified in terms of the binding agents employed as hydraulic cement bonded type, sodium silicate bonded type and, more recently, polyphosphate bonded type.
Cement bonded sands are more common in Europe than in the United States, and are used almost exclusively for large scale molding operations. Approximately 8 to 10 percent Portland cement is used, with a water-to-cement ratio of 0.4 which means that considerable setting time is required. In fact, strip times of 12-16 hours are normal, with approximately 24 hours being required between molding and pour-off. Recent developments have improved the setting characteristics by incorporating either cement accelerators or modified Portland cement compositions, which has provided strip times approaching 15-30 minutes, while the required time between molding and casting has been reduced to less than 4 hours. However, two major disadvantages are inherent in these systems which inhibit their use in the casting of high quality steels and high alloys. The cement accelerator systems rely on an organic catalyst derived from raw molasses which gives rise to carbon pick-up in the cast metal, while the modified Portland cement systems contain large amounts (e.g. 17%) of calcium sulfate which gives rise to potential carbon and/or sulfur contamination in the castings.
Calcium aluminate cements have been used for some time in castable refractories for ladle and furnace linings; their application to sand molding, however, is relatively new. See U.S. Pat. No. 3,600,203. Typical binder levels and strip times are similar to the modified cement compositions mentioned above but they are still limited by the high water of hydration contents. No information with respect to casting performance has been found in the literature.
Sodium silicate bonded sand mixes represent the largest share of the so-called inorganic mold sand compositions currently being utilized in steel and high alloy foundries. At least five systems based on sodium silicate binders have been proposed or are in commercial use, as shown by the technical literature and the patent art. The major difference between each system lies in the particular type of catalyst used in establishing the silicate bond. The systems are commonly known by the following names: (1) silicate/carbon dioxide process; (2) silicate/cement or fluid sand process; (3) silicon metal or ferrosilicon process, also sometimes referred to as the Nishiyama process; (4) silicate/sodium silicofluoride (hexafluorosilicate) process; and (5) silicate/ester process.
The sodium silicate/carbon dioxide process is the most widely used inorganic system available to the foundry industry, and considerable literature concerning its foundry use is available. See "The CO.sub.2.sup.- Silicate Process in Foundries", British Cast Iron Research Association, Westerham Press, Ltd. (1972). For many foundry operations, however, this system has disadvantages due to the carbon content accumulated in the mold after carbon dioxide gassing. In addition, the as-gassed molds exhibit poor humidity resistance and are susceptible to moisture absorption before pour-off under typically humid summer conditions. In addition, due to the gassing times required (60-90 seconds) the rate of mold production, with given molding equipment, is definitely limited.
The silicate/cement or fluid sand process utilizes the dicalcium silicate content of Portland cement or blast furnace slag to essentially dehydrate the sodium silicate, causing the sand mass to obtain gradual set. See "Sodium Silicate and Portland Cement as a Binder for Molding Sand", E. Leal, Steel Foundry Facts, February 1969, page 2; also "Innovations in Molding and Casting Process", P. F. Wieser, Steel Foundry Facts, February 1974, page 21. This system is similar to the conventional cement process in that long strip times are required and pour-off is not recommended before 12 to 16 hours. Since the initial water content is simply transferred to the cement and not removed this system also displays limitations similar to those of the conventional cement process with respect to water-related pour-off complications in steel castings.
The Nishiyama process utilizes silicon metal, ferrosilicon or calcium silicon fines to react with the water portion of the soluble sodium silicate, thereby essentially dehydrating the silicate and providing a cohesive bonding at low water contents. See "Silicon Metal Fines Used for Setting Sodium Silicate Bonded Steel Foundry Molds", Bates, Samco and Wallace, Steel Founders' Society of America, Report No. 61, January 1966; also Nishiyama U.S. Pat. No. 3,218,683. Several major disadvantages are found in commercial use of the process, one of the more important being the fact that a major by-product of the silicon metal reaction with water is the evolution of considerable quantities of hydrogen gas. This poses a potential fire and explosion hazard under foundry operating conditions. Also the silicon metal or ferrosilicon catalyst is prone to variations in its catalytic abilities, depending upon the initial water content, sizing, age and temperature of the sand mix. Another disadvantage is the high cost and poor availability of the finely powdered catalyst material required.
The sodium silicate/sodium silicofluoride process, like that just described, has a disadvantage due to evolution of fluorine gas at metal casting temperatures. The cost and availability of the sodium silicofluoride are again problems. Although the system is disclosed in an article entitled "Recent Developments in Self-Setting Sands", R. F. MacDonald, Steel Foundry Facts, February 1973, page 42, there is no literature reporting any production use of this particular system.
More recent developments in self-setting, sodium silicate sands rely upon weak acids or liquid organic materials as the gelling agent. See "Sodium Silicate as a Molding Media", A. F. Waller, Steel Foundry Facts, February 1973, page 48; "No Bake Molding and Core Process", T. A. Englate, Steel Foundry Facts, February 1975, page 14; and Palmer U.S. Pat. No. 3,881,947. These liquid organic materials consist mainly of poly-alcohol esters, which, being acidic, neutralize the sodium hydroxide phase of the silicate, causing gellation. See "Inorganic Self-Setting Binder Systems", J. M. Svoboda, Casting Metals Institute Short Course No. 201, 1975. Strip times are solely dependent on the type and not the amount of catalyst, with 10-15 minutes being a practical minimum strip time. While these systems are capable of developing the proper molding characteristics, they have the same two major disadvantages as that of the sodium silicate/carbon dioxide process. Carbon levels of the recommended sand/binder formulations are between 0.3 and 0.4 percent and consequently there is potential for migration of carbon from the mold at the interface with the metal during the casting process. Corrosion resistance and other properties of the casting can thus be adversely affected. In addition, the binder system under discussion is water soluble and generally results in as-set molds which exhibit poor humidity resistance, being prone to take-up of excess atmospheric moisture during the cure time following stripping from the pattern.
A more recent inorganic binder system based on a polyphosphate bond has become available. See "Performance Aspects of a New, Non-Silicate Inorganic No-Bake Binder for Foundry Mold and Core Applications", AFS Transactions, Vol. 74, page 463. Preliminary casting evaluations using such a binder system have indicated a phosphorus pick-up, and a potential to mold cracking leading to veining in the resulting castings. Of the several inorganic systems mentioned, this one is also the most expensive.
The practical foundry art has never employed, and with the possible exception noted below, has never suggested a binder system such as that forming the basis for the present invention, in which the reaction between a soluble silica source and a soluble alumina source in an alkaline medium is utilized to develop a suitable gel. The chemical reaction involving these two materials has, of course, long been known, more particularly as the basis for cation exchange systems in water softeners, and for forming molecular seives. In a publication entitled "Soluble Silicates, Their Properties and Uses" by James G. Vail, Chemistry, Vol. 1, page 236, Reinhold Publishing, 1952, there is a general discussion of "artificial zeolites" based on a gel obtained by mixing solutions of sodium silicate and sodium aluminate, but the publication contains no suggestion of any application to bonding particulate matter in preparing formed shapes. Vail was issued U.S. Pat. No. 2,131,338 directed to employing very dilute silicate/aluminate solutions as impregnating agents in preformed porous natural earth strata to cause gellation in situ. The purpose was to effect consolidation of the strata which would otherwise be unsuitable for supporting building foundations. The patent mentions in this connection that the gel has considerable water resistance and accordingly might be useful in impregnating "porous castings, cement blocks and sand molds"; but the patent gives no details in respect of any such suggested uses.
A report entitled "Steel Molding Sands Bonded with Sodium Silicate and Sodium Aluminate", Davis and Lownie, Journal of AFS, April, 1964, has been published on work done at the Battelle Memorial Institute Laboratories investigating the use of sodium silicate and sodium aluminate, also colloidal silica and sodium aluminate, both in foundry sand binder applications. The amount of aluminate used was quite low (0.027%), primary emphasis being laid on the silicate. No mention appears relative to any self-setting or no-bake mold/core making process. Casting trials showed scabbing and rattail defects with these mixes, and the conclusion reported was that the mixes were unsuitable for foundry use and no further work was recommended.
In a more recent publication entitled "Studies on Alkali Alumino-silicate Hydrogel", Mitra and Roy, Transactions of the Indian Ceramic Society, Volume XXXI (1972), pages 33-35, 52-56, 82-85 and 87-92, investigations are reported involving sodium silicate and sodium aluminate for the purpose of producing artificial zeolites. Gellation kinetics were studied and maximum gellation rates were determined in terms of mole ratios of alumina and silica in the aluminate and silicate solutions. While this work is quite definitive on the kinetics of the silicate/aluminate gel formation, and the study included silica/alumina mole ratio and concentration ranges that are now found useful for binder purposes in forming shapes of granular or particulate matter, the only work reported by the authors concerns that of forming gels for water treatment systems which involve totally different considerations. That is, the concept of coating granular particles and the associated problems inherent in self-setting, no-bake bonding requirements is foreign to anything considered by the authors, and the article accordingly gives no clues or suggestions as to possible application of the technology to the field of interest here discussed.
In addition to the foregoing technical literature, there is additional U.S. patent art directed specifically to binder systems for various mold and core mixes for foundry sands. Representative of the patent art thought to be most pertinent, in addition to the patents already mentioned, are the following.
U.S. Pat. Nos. 2,502,418; 3,600,203 and 3,874,885 deal generally with hydraulic cement type binders for mold or core mixes. The first of these employs a hydrated alkaline earth metal oxide and a water soluble alkali metal aluminate in a highly fluid slurry of granular material in order to allow it to be molded (in a filter press) to produce heat insulating blocks. The second patent utilizes a calcium aluminate cement plus a lithium chloride accelerating agent to improve the self-setting rate. In both cases, substantial amounts of water must be removed and the set times are long, usually at least an hour or more. The third patent in this group teaches incorporation of lithium carbonate as a catalyst, a source of carbon contamination in the finished casting if used in foundry mold/core making.
Several patents disclose refractory mixes utilizing alkali or alkaline earth metal aluminates without the addition of hydraulic cements. See for example U.S. Pat. Nos. 2,911,311; 3,017,677; 3,423,216 and 3,804,641. These however all employ various other adjuncts such as chlorides, silicone oil, ethyl silicate, ligno sulfonate, etc., and only the last of these appears to have any self-setting capability but is objectionable because of sulfur content. None incorporates any silicate.
U.S. Pat. No. 3,203,057 teaches combinations of silicates with aluminum oxide, but the insolubility of the oxide presents problems. The use of boronated aluminum phosphate binder systems (no silicates being present) is discussed in U.S. Pat. Nos. 3,923,525, 3,930,872 and 4,070,195. Phosphates as well as boron compounds however are potentially troublesome for steel and high alloy foundry applications on account of contamination of castings by migration at the mold surface. Cost, long strip times, non self-setting properties are additional problems with these systems.